Particular difficulties arise when welding the end plugs to the cladding tube of a fuel rod. The difficulties include the material of which the end plugs and the fuel rods are made, namely zirconium based alloys, such as Zircaloy-1, Zircaloy-2, Zircaloy-4, ZIRLO, ZIRLO-B, etc. Zirconium based alloys have a high disposition for oxidation at the melting temperature.
A further difficulty is that no atmospheric air or other surrounding gases are allowed to enter the interior of the fuel rod in connection with the final filling of hyperbaric helium, sealing and welding of the fuel rod. There are two methods of solving this problem. The fuel rod may be filled with fuel pellets and the rod be closed with end plugs. The end plugs are welded to the cladding tube in their final position without hyperbaric helium. Thereafter, fuel rod is filled with helium gas through a so called fill hole through one of the end plugs. The fill hole is then closed by means of a final welding operation. Alternatively, the fuel rod is filled with fuel pellets and helium gas prior to the final positioning and welding of the end plugs. The pressure prevailing in the fuel rods filled with helium is typically 5-10 bars for boiling water reactors, BWR, and 30-70 bars for pressurized water reactors, PWR. The ability to weld the end plug under these pressures in helium constitutes a further difficulty but eliminates the need for a filling hole and a welding step to close such a filling hole.
FR-A-2 625 022 discloses a method of welding an end plug to a cladding tube of a nuclear fuel rod. The known method comprises the steps of attaching a lower end plug, filling the interior of the fuel rod with fuel pellets and helium gas, positioning the upper end plug to abut the upper end of the cladding tube: at an interface, and applying a laser beam of a laser source. The proposed laser is a pulsed laser. The laser beam is directed to a welding zone at the interface to melt material of the end plug and the cladding tube at the interface.
U.S. Pat. No. 5,231,261 discloses a pulsed laser welding equipment for welding of fuel rods and monitoring the laser beam. The monitoring of the laser beam is done before the beam passes through the protective lens. It can therefore not identify changes to the protective lens e.g. from soot emanating from the welding process. The welding equipment setup for the girth welding is not done under pressure. The welding under helium pressure is done in a separate setup for a filling hole, thus not eliminating the need for such a filling hole and welding process step.
U.S. Pat. No. 5,958,267 discloses a method for welding fuel rods under high pressure and to control the laser position with a video system. The method claims to prevent soot accumulating on a laser window and to limit plasma formation. In practice this is difficult to achieve and the process, without control of the plasma, is possibly unstable.
U.S. Pat. No. 6,670,574 discloses a laser weld monitoring system for monitoring the welding of a pulsed laser beam. The system comprises two sensors, one sensor for sensing infrared radiation and one sensor for sensing reflection of the light of the laser beam. The method proposes essentially a trial and error method with a plurality of welds to correlate the complicated sensor curves to welding properties.
U.S. Pat. No. 5,651,903 also discloses a system for monitoring laser welding by means of an equipment comprising two sensors arranged beside the optical path of the laser beam. A first sensor senses infrared radiation of the temperature of the weld and a second sensor senses ultraviolet radiation of the plasma of the weld. The electrical signals are used to monitoring variations compared to a predetermined anomaly values obtained by empirical testing.
U.S. Pat. No. 6,710,283 discloses further laser weld monitoring system. The system comprises two sensors arranged beside the optical path of the laser beam. A first sensor senses reflected light of the laser beam and a second sensor, called a plasmatic sensor, senses the light emitted from welding zone. The method uses the frequency spectrum to compare actual variations in the signal with predetermined threshold values.